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Phenotype does not correlate with percent of mosaic cells except for trisomy 20. The presence of more than 60% of cells with trisomy 20 is associated with a poor prognosis.

All continuing pregnancies should have involvement of the medical genetics team.

Survivors with trisomy 16 have excellent postnatal catch-up growth and the majority have a good developmental outcome. Mosaic trisomies 8,9, and 14 have variable outcomes. Mosaic trisomy 20 has a normal outcome if thereare less than 60% abnormal cells.

CONDITION

In this chapter, information will be discussed regarding trisomies 8, 9, 14, 16, and 20. These are the trisomies that, if present in a mosaic form, are compatible with fetal survival at least until the end of the first trimester. They present with sonographic abnormalities or abnormalities of maternal serum screening analytes. All of these conditions are detected by cytogenetic analysis. Mosaic trisomies account for around 5% of the trisomies detected in human spontaneous abortions. In general, they are associated with advanced maternal age (James and Jacobs, 1996). For most of the trisomies, the general mechanism is nondisjunction in maternal meiosis I. This is then followed by a second mitotic error in chromosome division. The second division corrects the trisomy, but about a third of the time results in leaving behind two maternal copies of the particular chromosome. This is known as uniparental disomy (UPD). UPD can then cause an abnormal phenotype through homozygosity for recessive genes or if there are areas of imprinted genes on that particular chromosome (Reish et al., 1998). In contrast, current evidence seems to indicate that trisomy 8 has a different underlying mechanism. It appears to be due to postzygotic nondisjunction with a gain of a chromosome in some tissues.

Trisomies 8, 9, 14, 16 and 20 are generally lethal when present as a full trisomy. In this chapter only the mosaic trisomies will be discussed, as those are what is encountered clinically. For most of the mosaic trisomies, there is no relationship between the percent mosaicism and the clinical outcome, with the exception of trisomy 20.

Interestingly, all of the mosaic trisomies may present with postnatal skin pigmentation defects, such as hypomelanosis of Ito. The precise connection between mosaicism and the skin pigmentation defects is unknown. ...